The present invention relates to a breakaway joint for releasably connecting two components of a subsea oil or gas production system. More particularly, the invention relates to a load limiting breakaway joint which comprises one or more columnar members that are designed to buckle and allow the joint to separate under a predetermined load.
Subsea oil and gas fields are often produced using subsea well equipment which is located on the seabed and tied back to a surface production facility via a submarine flowline. The flowline from the surface production facility is usually terminated near the subsea well equipment and then connected to the well equipment with a flowline jumper. Due to the typically great length of the flowline, a significant risk exists that the flowline may be snagged and dragged, for example, by an anchor from a vessel which is blown off location during a storm. If the flowline is dragged, substantial tension, bending and torsion stresses can develop at the connection between the jumper and the well equipment which may damage the well equipment. Thus, a need exists to protect the well equipment from damage in the event the flowline is snagged.
One prior art device that has been used to protect the well equipment in such circumstances is a load limiting breakaway flange joint. One or more such flange joints are commonly installed in the jumper, and each flange joint is designed to break away at a predetermined load, which is typically a small fraction of the breaking strength of the flowline. However, such flange joints usually do not function well when subjected only to tensile loads. In order for the flange joint to break away at a reasonably small predetermined load, it must be subjected to a fairly specific combination of torsion and bending stresses. To achieve such combination of stresses, awkward and difficult to install jumper configurations must often be used, and the well equipment must sometimes be arranged on the seabed in a specific orientation relative to the flowline, which can result in sub-optimal field layouts. In addition, the normal variations of the yield strength and ultimate strength of the flange joint material can lead to significant variations in the breakaway load.
Another prior art device which is used to protect the well equipment in the event the flowline is snagged is a load limiting pressure balanced joint. However, such joints are dependent on dynamic elastomer seals to balance the end thrust loads created by the internal pressure within the flowline or jumper, and such elastomer seals degrade over time and therefore pose potential leak paths. Also, due to the large size and weight of these joints, they must often be installed in the flowline rather than the jumper. When so located, the pressure balanced joint is subjected to the dynamic loads created when the flowline expands and contracts due to pressure and temperature cycles, which could potentially result in a premature breakaway. In addition, these joints employ shear pins or shear rings as the triggering mechanism for the breakaway feature, and such shear type devices often exhibit a wide range of failure loads due to the variability of the shear strength of the material and the tolerances of the working parts.
In accordance with the present invention, these and other disadvantages in the prior art are overcome by providing a breakaway joint for releasably connecting a first component to a second component. The breakaway joint comprises a first member to which the first component is attached, a second member to which the second component is attached and a plurality of columnar members each having first and second ends. The first ends of the columnar members are coupled to the first member, and the breakaway joint further comprises a latching mechanism which is engaged by both the second member and the second ends of the columnar members. The latching mechanism latches the first member to the second member and converts a tensile load acting between the first and second members into a compressive load acting on the columnar members. Thus, when the tensile load reaches a predetermined level, the columnar members will buckle and the latching mechanism will release the first member from the second member.
In accordance with the preferred embodiment of the invention, the first member comprises a receptacle and the second member comprises an outer surface having a portion which is slidably received in the receptacle. This engagement between the first and second members helps the breakaway joint to resist bending loads and thereby prevents the bending loads from causing the breakaway joint to separate prematurely.
In one embodiment of the breakaway joint of the present invention, the latching mechanism comprises a flange which is slidably mounted on an outer surface of the first member and a plurality of elongated members each having a first end which is connected to the flange and a second end which is connected to the second member. In addition, the second ends of the columnar members are engaged by the flange. Thus, the tensile loads acting between the first and second members are reacted through the elongated members and the flange to the columnar members, and when the columnar members buckle, the first member is allowed to separate from the second member.
In accordance with another embodiment of the invention, the receptacle of the first member comprises a first radial groove formed therein, the outer surface of the second member comprises a second radial groove which is located opposite the first radial groove when the second member is received in the first member, and the latching mechanism comprises a latch ring having radially outer and inner portions which are disposed in the first and second grooves, respectively, to connect the first member to the second member. In addition, the second ends of the columnar members are coupled to the latch ring to maintain the latch ring in the first and second grooves. Thus, the tensile loads acting between the first and second members are reacted though the second groove and the latch ring to the columnar members, and when the columnar members buckle, the latch ring will move into the first groove and allow the first member to separate from the second member.
In accordance with yet another embodiment of the invention, the receptacle of the first member comprises a first radial groove formed therein, the outer surface of the second member comprises a second radial groove which is located opposite the first radial groove when the second member is received in the first member, and the latching mechanism comprises a latch ring having radially outer and inner portions which are disposed in the first and second grooves, respectively, to connect the first member to the second member. The latching mechanism also comprises a reaction ring disposed in the first groove against the radially outer portion of the latch ring to maintain the radially inner portion of the latch ring in the second groove. In addition, the second ends of the columnar members are coupled to the reaction ring to maintain the reaction ring against the latch ring. Thus, the tensile loads acting between the first and second members are reacted though the second groove, the latch ring and the reaction ring to the columnar members, and when the columnar members buckle, the latch ring will move into the first groove and allow the first member to separate from the second member.
The columnar members are preferably designed to behave as slender columns. Accordingly, the axially compressive critical load required to cause the columnar members to collapse by bucking depends only on the length and the diameter of the columnar members and the modulus of elasticity, or Young""s modulus, of the material of the columnar members. Since the length and diameter of the columnar members can be accurately controlled and the modulus of elasticity of the columnar members is highly consistent for a given material, the columnar members can be designed to have an accurately predictable critical load. Moreover, the critical load for the columnar members is independent of the yield strength and the ultimate strength of the material, which can vary widely. In addition, the critical load required to initiate buckling in the columnar members is greater than the load required to further collapse the columnar members. Therefore, once the columnar members are subjected to the predetermined critical load, they will buckle and collapse.
Thus, when installed in a flowline or a jumper which is connected to the well equipment, the breakaway joint of the present invention will effectively release the flowline from the well equipment in the event the flowline in snagged. Since the tensile load caused by the snag is transmitted to the columnar members, the breakaway joint will separate under a relatively predictable predetermined load. Furthermore, since the breakaway joint is particularly effective under tensile loads, no need exists to orient the jumper or the well relative to the flowline in order to induce torsion and bending stresses in the breakaway joint. In addition, the breakaway joint is relatively insensitive to compressive thermal loads that have complicated the design of certain prior art devices.
These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments.